The present invention relates to an alkaline storage battery, more specifically an alkaline storage battery using a paste-type electrode and a method for producing the paste-type electrode for use in the alkaline storage battery.
With the advancement of handy communication equipment and personal computers, the scale of the market of alkaline storage batteries used for those apparatuses has been expanded. Recently, the demand for a light-weight and high capacity battery has been increased rapidly in those fields of art. Moreover, there is also an increasing demand for the alkaline storage battery as the power source for use in electric tools and auxiliary power sources for charging and discharging at a large current.
Methods for producing electrodes for use in alkaline storage batteries are roughly classified into two types; one is the process of producing an electrode called "sintered-type electrode" by applying a paste of a nickel powder kneaded with a viscosity improver onto the surface of a conductive core material called "punched metal", which is a perforated nickel plate or the like, then sintering the paste-applied conductive core material to form a sintered porous substrate, and impregnating it with an active material. The other is the process of forming the paste-type electrode by filling or applying a paste containing an active material into or onto a porous metal substrate such as foamed metal, nonwoven nickel cloth or the like, or a conductive core material such as punched metal, expanded metal or the like.
There is a proposed method for producing a substrate by implanting fibrous nickel on a wire stainless steel, compressing and sintering the whole to form a sintered substrate (Japanese Laid-Open Patent Publication Sho 61-293618). This method intends to solve the inconveniences of crack development in the sintered substrate and difficult control of the substrate thickness involved in the sintered-type electrode.
There is another proposal to use a carbon fiber duster composed of a piled basement layer including carbon fibers and a nap-raised part formed by napping the basement layer as a conductive material (i.e., substrate) for the electrode used in secondary batteries, particularly in a sodium-sulfur battery (Japanese Laid-Open Patent Publication Hei 8-144153).
As far as the nickel electrode is concerned which has poor active material conductivity, it includes a porous metal such as foamed metal, nonwoven nickel cloth, or the like as the substrate of the paste-type electrode. Those porous metal substrates exhibit poor charge/discharge characteristics at a large current because of a long course of current collection from the active material to the terminal serving as the current input/output port, compared to the sintered-type electrode with an arrangement of the conductive core material in the center of the substrate along its thickness. Furthermore, these substrates are low in substrate strength and active material retention, because they have, as a whole, large pore sizes compared to the sintered-type electrode. In the nickel electrode, repeated charging/discharging operations cause a significant change in the volume of the active material. At the same time, swelling of the electrode plate occurs due to absorption of an electrolyte into the electrode. Thus, in such electrode with low active material retention, electric contact between the substrate and active material particles is apt to be impaired, causing a significant deterioration of current collection of the electrode.
On the other hand, in a cadmium electrode and a hydrogen storage alloy electrode which have relatively high active material conductivity, two-dimensional core materials such as punched metal are used as the substrates. Such electrodes have been widely used that are prepared by applying a mixture of an active material with a conductive material, such as carbon powder or carbon fiber, for supplementing conductivity, and a binder for supplementing active material retention, onto a conductive core material. However, even those electrodes with an addition of a conductive material exhibit insufficient current collection when they are charged and discharged at a large current.
The use of two-dimensional conductive core materials such as punched metal in the nickel electrode has also been studied for long for reducing the manufacturing cost of electrodes. However, since no binders and conductive materials suitable for the nickel electrode have not been found to date, the nickel electrode remains poor in charge/discharge characteristics and cycle life characteristic by repeated charge/discharge operations. Under the situation, nickel electrodes using the two-dimensional conductive core material have not been put to practical use industrially.
The sintered-type electrode has better charge/discharge characteristics at a large current than the paste-type electrode, but the substrate used for the former has a low porosity compared to the porous substrate used for the latter, and is difficult to reduce its thickness. Therefore, the sintered-type electrode is lower in capacity per unit volume than the paste-type electrode. Furthermore, the sintered substrate has a drawback such that due to its smaller pore size than that of the paste-type electrode, it becomes necessary to impregnate the substrate with an active material salt solution several times before filling of a required amount of an active material into the substrate, making the manufacturing process time-consuming and laborious.